Effect of Environmental Element on Porosity of CRUD in PWR Fuel Cladding

Abstract

In this study, controlled experiments were conducted for different heat flux, metal ion concentration and exposure time to analyze the effect of those on CRUD structure—including porosity, chimney distribution, and chemical composition. To investigate the growth mechanism of CRUD and establish a database for validating a CRUD prediction model, simulated CRUD deposition experiments were conducted under PWR primary water condition. Structural analysis was performed using cross-sectional images obtained with Scanning Electron Microscopy combined with Focused Ion Beam (SEM-FIB) equipment, allowing quantification of CRUD thickness, chimney distribution, and porosity. The chemical composition of the CRUD layer was analyzed from cross-sections using Energy Dispersive X-ray Spectroscopy (EDS) integrated with SEM-FIB. Detailed microstructural information was obtained through X-ray Diffraction (XRD) analysis. From these results, a CRUD growth mechanism influenced by the environmental parameter (heat flux, metal ion concentration and time) was proposed. The mass flux from the bulk coolant to the CRUD layer has a critical effect on CRUD deposition, including increases in CRUD thickness and decreases in porosity. As the CRUD layer grows and its porosity decreases, the internal temperature rises, altering the redox environment within the layer and provoking the deposition of nickel oxide or nickel metal. However, the growth of CRUD and the decrease in porosity reach a limit over time. Wick boiling plays a crucial role in CRUD deposition under varying heat flux conditions. The porous CRUD layer, characterized by chimney structures, facilitates a unique boiling phenomenon where capillary action draws coolant into the layer. Upon contacting the hot cladding surface, the coolant boils and vaporizes. The steam escapes through the chimneys due to buoyancy and density difference, creating a pressure gradient that enhances further coolant influx through the porous medium. This proposed mechanism illustrates that heat flux influences not only the amount of CRUD deposited but also its structural and chemical characteristics. Higher heat flux accelerates chimney formation and wick boiling, leading to denser, nickel-rich CRUD layers with lower porosity. The time- dependent observations confirm that the development of chimney structures and the transition from porosity reduction to thickness growth are critical aspects of CRUD formation A CRUD porosity model was developed and validated to better understand CRUD deposition and behavior in PWR environments. Key parameters influencing CRUD formation, including heat flux, metal ion concentration, and exposure time, were identified through a literature review. Experimental results showed CRUD porosity inversely correlates with these parameters, significantly impacting heat and mass transfer in nuclear fuel cladding. Mass flux from bulk coolant to CRUD layer increases with higher heat flux and metal ion concentrations, leading porosity decrease. The change in porosity was shown to demonstrate a self-feedback, proportional to the porosity value itself, resulting in a decrease in porosity over time. A CRUD porosity model based on mass flux (combination of heat flux and metal ion concentration) and exposure time was proposed, referencing the MAMBA-3D model. The model was validated using CRUD analysis data from US PWR plants and WALT simulated CRUD experiment data, confirming the model's accuracy within an approximately 20% error margin. By enhancing the predictive accuracy of CRUD deposition effects on cladding surfaces, this model can contribute to better managing the operational performance and safety of PWRs, addressing critical issues like AOA, CILC, and radioactivity control. This advancement marks a significant step forward in understanding and mitigating the impacts of CRUD in nuclear reactors.